Optional Characters
     Must have Knuckles for Cream and a Flying Character for both.

Cream the Rabbit: 
     After  having  Knuckles join your team, go to Cream in the center
of  the Green Hill Zone. Go find Cheese in the tower to the South East
and she'll join.

E-123 Omega: 
     Complete  the  side  quest  in  the North East of Metropoplis and
acend  to  the  top.  Fly  to  the  West  with Tails and he'll be in a
underground room.

New game plus
     Complete the game once.